Google Trends is a fantastic way to look for hot topics for your hubs. Not only can you see if it's trending up or down over time, you can also see trends for individual countries and states.
Don't forget to use the filters to see not only the trend over time but look to see what the trend is over the last year.
If you look carefully at the graphs you should also be able to tell if your topic is seasonal and know when the best time is to publish your hub.
Another tool you may wish to look at is Yahoo! Buzz Log.